1. Field of the Invention
This invention relates to a group III-V nitride-based semiconductor substrate, a group III-V nitride-based device and a method of fabricating the same. In particular, this invention relates to a group III-V nitride-based semiconductor substrate having a high-quality AlN layer with a good crystalline quality or a similar layer with a high thermal conductivity formed on its surface, a high-output group III-V nitride-based device with the same substrate to provide an excellent heat radiation property, and a method of fabricating the substrate or the device with a good reproducibility and at a low cost.
2. Description of the Related Art
Group III-V nitride-based compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN) and aluminum gallium nitride (GaAlN) are used to fabricate a short-wavelength light emitting device, especially, blue light emitting diode (LED) since they have a wide bandgap and are of direct transition type in interband transition. Recently, a ultraviolet LED with a further short wavelength and a white LED with a phosphor combined with such an LED are in practical use. Furthermore, the group III-V nitride-based compound semiconductors have applied to electronic devices and power devices since they have a good heat resistance and environment resistance.
In fabricating a semiconductor device, an underlying substrate is generally used which has the same lattice constant or linear expansion coefficient as a crystal to be grown on the substrate, so as to conduct the homo-epitaxial growth. For example, a GaAs single crystal substrate is used to conduct the epitaxial growth of GaAs, AlGaAs.
However, for group III-V nitride-based semiconductor crystals, no group III-V nitride-based semiconductor substrate with a practical size and characteristics has been obtained thus far. Therefore, most of nitride-based LED's developed are fabricated by using the hetero-epitaxial growth where group III-V nitride-based semiconductor crystals are grown on a sapphire substrate with a near lattice constant by MOVPE (metalorganic vapor phase epitaxy). Many problems arise from the hetero-growth.
For example, due to the difference in linear expansion coefficient between the sapphire substrate and GaN, a problem has occurred that the substrate after the epitaxial growth is warped significantly. This causes a reduction in yield since the substrate may crack during the photolithography or chip fabrication process after the epitaxial growth.
Further, since the lattice constant of the sapphire substrate is different from the GaN, a buffer layer needs to be deposited thereon at lower temperature than the proper crystal growth temperature before growing the nitride single crystal. This is a factor to increase the time required for the crystal growth. Furthermore, in case of the growth on the sapphire substrate, dislocations are generated as many as 108 to 109/cm−2 in the GaN epi-layer due to the difference in lattice constant between the sapphire substrate and the GaN. The dislocation is a factor to lower the output and reliability of LED. Although in the conventional blue LED's the dislocation is not so much questioned, the influence of the dislocation on the device characteristics is assumed to increase hereafter according as the output of LED is further increased or as the emission wavelength thereof is shortened to develop the ultraviolet LED. Therefore, some measure is needed to reduce the dislocation.
In order to solve these problems, a self-standing substrate of GaN single crystal has been developed in recent years. The GaN self-standing substrate is produced by, e.g., ELO (epitaxial lateral overgrowth), where a mask with openings is formed on the underlying substrate (=sapphire substrate), and then the lateral growth is conducted through the openings to obtain GaN with a reduced dislocation, and after forming the GaN layer on the sapphire substrate, the sapphire substrate is removed by etching to obtain the GaN self-standing substrate (e.g., JP-A-11-251253).
As a progress of the ELO, FIELO (facet-initiated epitaxial lateral overgrowth) has been developed (e.g., Akira Usui et al., “Thick GaN Epitaxial Growth with Low Dislocation Density by Hydride Vapor Phase Epitaxy”, Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L899-902). The FIELO is in common with the ELO on the point that the selective growth is conducted by using the silicon dioxide mask, but it is different from the ELO on the point that a facet is formed at the mask opening during the selective growth. By forming the facet, the propagation direction of dislocation is changed so that the threading dislocation reaching the top surface of the epitaxial growth layer can be reduced. When a thick GaN layer is grown by using the FIELO on an underlying substrate such as sapphire and then the underlying substrate is removed, the GaN self-standing substrate with relatively few defects can be obtained.
The other methods for obtaining a low-dislocation GaN self-standing substrate include DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: e.g., Kensaku Motoki et al., “Preparation of Large Freestanding GaN Substrates by Hydride Vapor Phase Epitaxy Using GaAs as a Starting Substrate”, Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L140-L143, and JP-A-2003-165799). The DEEP is conducted such that GaN is grown on a GaAs substrate, which is removable by etching, by using a SiN patterning mask while intentionally forming pits surrounded by facets on the surface of crystal, accumulating dislocations at the bottom of pits to allow regions other than pits to have a low dislocation density.
Further, the other methods for obtaining a low-dislocation GaN self-standing substrate include the method that a GaN layer is formed on a sapphire C-face ((0001) facet) substrate, a titanium film is formed thereon, the substrate is then subjected to heat treatment in an atmosphere of hydrogen gas or hydrogen-containing compound gas to form voids in the GaN layer, and a GaN semiconductor layer is formed on the GaN layer e.g., JP-A-2003-178984).
The GaN self-standing substrate obtained by using the ELO or DEEP etc., where the GaN film is grown on the hetero-substrate by HVPE and then the GaN layer is separated from the underlying substrate, is used mainly for the development of laser diode (LD) which requires especially a low-dislocation crystal. However, recently, it is also used for the LED substrate.
The GaN substrate has a high thermal conductivity of 1.98 W/(cm·k) as compared to that (0.42 W/(cm·k)) of the conventional sapphire substrate. Since the heat radiation efficiency is thus enhanced, the development of a high-output light emitting device or power device with a large heat generation has been accelerated.
According to this trend, the next demand is directed to a self-standing substrate of AlN which has a higher thermal conductivity (3.2 W/(cm·k) in theoretical value) than the GaN. Thus, the growth of Al-containing group III-V nitride-based semiconductor crystal has been researched by using HVPE which is employed to grow the GaN substrate (e.g., Andrey Nikolaev et al., “AlN Wafers Fabricated by Hydride Vapor Phase Epitaxy”, MRS Internet J. Nitride Semicond. Res. Volume 5S1 Published 2000, W6.5., and Y. Kumagai et al., “Hydride vapor phase epitaxy of AlN: the rmodynamic analysis of aluminum source and its application to growth”, Phys. Stat. Sol. (c), Vol. 0, No. 7, 2003, pp. 2498-2501).
Further, a GaN stacked substrate is known in which a Si—Al0.05Ga0.95N layer is grown about 10 μm on a GaN substrate (JP-A-2005-191306), and an AlGaN-based composite substrate is known in which an Al0.15Ga0.85N upper substrate is grown 300 to 400 μm on a GaN lower substrate (JP-A-2005-277015).
However, when the AlN is grown by HVPE as disclosed in Andrey Nikolaev et al. and Y. Kumagai et al., aluminum chloride as a raw material may react with a quartz member composing the reactor depending on the grow conditions to corrode the member. Thus, when a thick film of AlN is needed to grow, the quartz member will be corroded so much. Therefore, the quartz member needs to be replaced frequently, the manufacturing cost will be increased, and the quartz member may be broken during the crystal growth.
In addition, since a bulk of AlN is difficult to grow, the technique for producing a high-quality AlN substrate with a good reproducibility is not realized at present.
On the other hand, when the AlGaN is in a separate process formed on the GaN substrate made previously as disclosed in JP-A-2005-191306 and JP-A-2005-277015, native oxide film will be generated since the GaN substrate is once exposed into the air, or the growth surface may be not clean due to a contamination during the handling. Further, since the GaN surface is subjected to a rising and falling temperature cycle, it may be denatured by heat. As a result, a layer containing many defects may be generated at the GaN/AlGaN interface, and a number of cracks may be generated in the AlGaN crystal layer.